#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <assert.h>

char* my_strcat(char* destination, const char* source) {
	char* ret = destination;
	assert(destination);
	assert(source);

	while (*destination) {
		destination++;
	}

	while (*destination++ = *source++) {
		;
	}
	return ret;
}

int main() {
	char arr1[] = "abcdef";
	char arr2[20] = "new_";
	char* ret = my_strcat(arr2,arr1);

	printf("%s", ret);
}